It's really easy to order a service from us, select a category, display the offer, and click on the cart icon (for example: Shared base, 60 euro) and insert the domain name.
Inside the cart you can choose if it's a new domain name or a transfer of an already existing domain. Afterwards enter the personal details of the person who is (or will be) the owner of the domain.
You can also order muliple hostings putting them in the same cart, a tab will open for every domain name but the order will be just one.

How much time between the order and the registration? 

It depends on  the extension, the hosting will be ready from one to three days. 
Once you pick the perstashop version you prefer, servermac will install (or transfer) it for free.